There are many applications in which it is desired to monitor a condition at a remote location. Examples include measuring temperature, pressure, humidity, gas concentration, flow rate and mechanical stresses in meteorology, civil engineering, mining, oil refining, natural gas and oil production and others.
By far the most satisfactory method of monitoring conditions at remote locations is to position at the remote location an electrically driven sensor unit which is provided with means to transmit to a central location a signal representative of the parameter in question. The signal may be transmitted by wireless communication, but it is generally simpler and less expensive to transmit by cable. The provision of a cable between the central monitoring station and individual sensors also enables the sensors to be driven from power supplied from a central unit rather than needing each to be supplied with its own power generation or storage system.
Detector and sensor units are often dependent for their accuracy on a stabilised power supply. This gives rise to difficulties particularly where long cable runs are involved since these are subject to voltage drop and other fluctuations and it is frequently necessary to provide an individual stabilising unit at each remote location.
Conventional series regulators are very inefficient, particularly when designed to operate over a large input voltage range. This power loss may make system design difficult or impossible in cases where the inherent power consumption of the sensor device is not much less than the maximum power which may be transmitted through the protective zener barriers which are fitted to the supply lines of electrical equipment used in hazardous areas. In such cases, it may be desirable or even essential to use an efficient voltage regulator of the "switched mode" type. Switched mode voltage regulators, often referred to simply as "switching regulators" are described in, e.g. R. S. Olla "Switching regulators: the efficient way to Power" Electronics, 16th Aug. 1973 at page 91 ff.
In systems of this kind, it is desirable, for economic reasons, that the cable carries both power and signal from the sensor to the central unit. Variable frequency signalling over a frequency range of e.g. 10 to 20 kHz or higher constitutes an acceptable and widely used system, but if this is combined with switched mode voltage regulators interference may be caused by harmonics of the basic stabiliser frequency transmitted on the cable. These disadvantages can in some cases be overcome by sophisticated filtering techniques but such techniques are themselves bulky and expensive.